weights and measures, units and standards for expressing the amount of some quantity, such as length, capacity, or weight; the science of measurement standards and methods is known as metrology.

Crude systems of weights and measures probably date from prehistoric times. Early units were commonly based on body measurements and on plant seeds or other objects from agriculture. As civilization progressed, technological and commercial requirements led to increased standardization. For example, because the length of the human foot or the width of the palm varies from individual to individual, it probably became necessary first to specify a particular individual (e.g., the king) and later to reproduce standards based on this commonly accepted unit of length. Units were usually fixed by edict of local or national rulers and were subdivided and multiplied or otherwise arranged into systems of measurement.

Standards varied greatly in different localities, although conquest and trade stimulated some correspondence between systems, e.g., between the systems of Egypt, Babylon, and Phoenicia. A high degree of standardization was achieved in the Roman Empire, but after its fall considerable diversity returned. The foot, which was one of the earliest units, is believed to have had as many as 280 variants in Europe as late as the 18th cent. Today the chief systems are the English units of measurement and the metric system.

The United States is one of the few countries still using the English system; all other major nations have either converted to the metric system or committed themselves to conversion. The English system is much older and less practical than the metric system, and in the United States there has been considerable discussion in favor of adopting the metric system as the principal system. However, attempts to legislate such a change in the U.S. Congress have failed. The basic units of the English system, the yard of length and the pound of mass, are now defined in terms of the metric standards, the meter of length and the kilogram of mass.

Before 1960 the meter was defined as the distance between two scratches on a prototype bar kept at the International Bureau of Weights and Measures (est. 1875) at Sèvres, France, near Paris. In 1960 it was redefined in terms of an atomic standard. This new standard is more stable than the old meter bar, is indestructible, and is easily reproduced, eliminating the need for periodic comparison with a single standard. The kilogram is defined in terms of a prototype cylinder kept at the bureau.

In the United States, Congress has the constitutional right to fix standards, but except for purposes of customs and internal revenue, weights and measures legislation has been, for the most part, permissive. Sets of official weights and measures were sent to the states in 1856, but legislation and enforcement are largely state prerogatives. The federal government permitted the use of the metric system in 1866 and established a conversion table based on the yard and the pound; in 1893 the yard and the pound were redefined in terms of the metric prototypes of the meter and the kilogram. The major arguments against total conversion to the metric system in the United States are that it would involve great expense in industry and would cause widespread confusion among the general public.

See the table entitled Common Weights and Measures.

measures: see weights and measures.

U.S. Customary System of weights and measures: see English units of measurement.

Length

Metric System

1 millimeter = 1/1,000 meter

1 centimeter = 1/100 meter

1 decimeter = 1/10 meter

1 meter (basic unit of length)

1 dekameter = 10 meters

1 kilometer = 1,000 meters

American and British Units

1 inch = 1/36 yard = 1/12 foot

1 foot = 1/3 yard

1 yard (basic unit of length)

1 rod = 51/2 yards

1 furlong = 220 yards = 40 rods

1 mile = 1,760 yards = 5,280 feet

1 fathom = 6 feet

1 nautical mile = 6,076.1 feet

Conversion Factors

1 centimeter = 0.39 inch

1 inch = 2.54 centimeters

1 meter = 39.37 inches

1 foot = 0.305 meter

1 meter = 3.28 feet

1 yard = 0.914 meter

1 meter = 1.094 yards

1 kilometer = 0.62 mile

1 mile = 1.609 kilometers

Area

Metric System

1 square centimeter = 1/10,000 square meter

1 square decimeter = 1/100 square meter

1 square meter (basic unit of area)

1 are = 100 square meters

1 hectare = 10,000 square meters = 100 ares

1 square kilometer = 1,000,000 square meters

American and British Units

1 square inch = 1/1,296 square yard = 1/144 square foot

1 square foot = 1/9 square yard

1 square yard (basic unit of area)

1 square rod = 301/4 square yards

1 acre = 4,840 square yards = 160 square rods

1 square mile = 3,097,600 square yards = 640 acres

Conversion Factors

1 square centimeter = 0.155 square inch

1 square inch = 6.45 square centimeters

1 acre = 0.405 hectare

1 hectare = 2.47 acres

1 square kilometer = 0.386 square mile

1 square mile = 2.59 square kilometers

Volume and Capacity (Liquid and Dry)

Metric System

1 cubic centimeter = 1/1,000,000 cubic meter

1 cubic decimeter = 1/1,000 cubic meter

1 cubic meter = 1 stere (basic unit of volume)

1 milliliter = 1/1,000 liter = 1 cubic centimeter

1 centiliter = 1/100 liter

1 deciliter = 1/10 liter

1 liter = 1 cubic decimeter (basic unit of capacity)

1 dekaliter = 10 liters

1 hectoliter = 100 liters = 1/10 cubic meter

American and British Units

1 cubic inch = 1/46,656 cubic yard = 1/1,728 cubic foot

1 cubic foot = 1/27 cubic yard

1 cubic yard (basic unit of volume)

1 U.S. fluid ounce = 1/128 U.S. gallon = 1/16 U.S. pint

1 British imperial fluid ounce = 1/160 imperial gallon = 1/20 imperial pint

1 pint = 1/8 gallon = 1/2 quart

1 quart = 1/4 gallon

1 U.S. gallon (basic unit of liquid capacity in the United States) = 231 cubic inches

1 imperial gallon (basic unit of liquid capacity in some Commonwealth nations) = 277.4 cubic inches

1 dry pint = 1/64 bushel = 1/2 dry quart

1 dry quart = 1/32 bushel = 1/8 peck

1 peck = 1/4 bushel

1 U.S. bushel (basic unit of dry capacity in the United States) = 2,150.4 cubic inches

1 imperial bushel (basic unit of dry capacity in some Commonwealth nations) = 2,219.4 cubic inches

Conversion Factors

1 cubic centimeter = 0.06 cubic inch

1 cubic inch = 16.4 cubic centimeters

1 cubic yard = 0.765 cubic meter

1 cubic meter = 1.3 cubic yards

1 milliliter = 0.034 fluid ounce

1 fluid ounce = 29.6 milliliters

1 U.S. quart = 0.946 liter

1 liter = 1.06 U.S. quarts

1 U.S. gallon = 3.8 liters

1 imperial gallon = 1.2 U.S. gallons = 4.5 liters

1 liter = 0.9 dry quart

1 dry quart = 1.1 liters

1 dekaliter = 0.28 U.S. bushel

1 U.S. bushel = 0.97 imperial bushel = 3.5 dekaliters

Weight (Mass)

Metric System

1 milligram = 1/1,000,000 kilogram = 1/1,000 gram

1 centigram = 1/100,000 kilogram = 1/100 gram

1 decigram = 1/10,000 kilogram = 1/10 gram

1 gram = 1/1,000 kilogram

1 dekagram = 1/100 kilogram = 10 grams

1 hectogram = 1/10 kilogram = 100 grams

1 kilogram (basic unit of weight or mass)

1 metric ton = 1,000 kilograms

American and British Units: Avoirdupois

1 grain = 1/7,000 pound = 1/437.5 ounce

1 dram = 1/256 pound = 1/16 ounce

1 ounce = 1/16 pound

1 pound (basic unit of weight or mass)

1 short hundredweight = 100 pounds

1 long hundredweight = 112 pounds

1 short ton = 2,000 pounds

1 long ton = 2,240 pounds

American and British Units: Troy and Apothecaries'

1 grain = 1/7,000 avoirdupois pound = 1/5,760 troy or apothecaries' pound

1 apothecaries' scruple = 20 grains = 1/3 dram

1 pennyweight = 24 grains = 1/20 troy ounce

1 apothecaries' dram = 60 grains = 1/8 apothecaries' ounce

1 troy or apothecaries' ounce = 480 grains = 1/12 troy or apothecaries' pound

1 troy or apothecaries' pound = 5,760 grains = 5,760/7,000 avoirdupois pound

Conversion Factors

1 milligram = 0.015 grain

1 grain = 64.8 milligrams

1 gram = 0.035 avoirdupois ounce

1 avoirdupois ounce = 28.35 grams

1 troy or apothecaries' pound = 0.82 avoirdupois pound = 0.37 kilogram

1 avoirdupois pound = 1.2 troy or apothecaries' pounds = 0.45 kilogram

1 kilogram = 2.205 avoirdupois pounds

1 short ton = 0.9 metric ton

1 metric ton = 1.1 short tons

British Imperial System of weights and measures: see English units of measurement.